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Supersonic pneumatic

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: DYI » Sun Sep 09, 2007 3:57 pm

Hybrids only complicate the matter, as you then need an ignition source, a fuel metering device, fuel, some accurate gauges, some means of venting the chamber, and a spark gap.

Hooking this thing up to mains power would likely result in my gruesome demise, due to my complete lack of knowledge of electronics, and the way that any simple electric device I make tends to either short circuit, die violently, or slowly degrade for no apparent reason to the point that it is useless.

I know that I can't get anything like Mach 2 with this like you can with hybrids, I just want to see if I can get slightly over the speed of sound, with no heating involved. It seems as though some commercial, non-springer air rifles can do this, so I should be able to too.
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Unread postAuthor: dongfang » Sun Sep 09, 2007 4:13 pm

Hi,

OK, don´t get yourself fried ;) ;) ;) , it would be a waste of good electrons, and contribute to global warming. And, we´d miss you here.

Well for heating gas I can only really think of internal power and external power. Burning or heating. I would like to suggest hypergolic fuels but I know of none that aren´t extremely poisonous. Or a monopropellant hydrazine / catalyzer rocket, infeasible for the same reason (could have been cool). I think concentrated hydrogen peroxide can do some interesting stuff through a catalyzer too, but again it is extremely dangerous.

I think it's a very interesting challenge you gave yourself there. I did it with an unpressurized combustion a little over a year ago, if anybody remembers.

OK, one idea: Use a standard pneumatic set-up with a large bore. Instead of a projectile, the pneumatic will power a piston, that again pushes another piston (maybe the two are just the two sides of one piston; maybe they have different diameter). The second piston is a pump ... in the other end of it´s cylinder is the burst disk and the projectile.

The simple idea here is that the air driving the projectile will have no time to cool down between compression and decompression. And, it will certainly be warmer / hotter in the barrel than before compression (= ambient temperature).

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Unread postAuthor: DYI » Sun Sep 09, 2007 6:18 pm

I thought of a similar idea to that before, but the piston would be moving very quickly, with a hell of a lot of energy, and I could see it completely destroying whatever it runs into. It definitely could go supersonic though.

Any ideas on stopping this hypothetical piston which is moving at hundreds of fps, with more than 50 ft/lbs of energy without destroying it or the bumper? The piston itself would have to be pretty tough for this to work, but also very low density. And I really don't want this turning into a 3 stage gun that takes 20 minutes to reload.

Maybe forcing the air from a large diameter hole through a much smaller one could accelerate it past the speed of sound? Like, using a massively oversized valve?
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Unread postAuthor: scatdawg » Sun Sep 09, 2007 7:59 pm

dyi, i have been intersted in this idea of a very high powered pneumatic for a long time. if there is anything i can do to help you as far as making any metal based parts please let me know. i would donate my time and expertise. i am a machinist with access to cnc lathes and mills.
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Unread postAuthor: Lentamentalisk » Sun Sep 09, 2007 9:45 pm

Maybe forcing the air from a large diameter hole through a much smaller one could accelerate it past the speed of sound? Like, using a massively oversized valve?


this idea should work, supposing that you have quite a long barrel. Your actual PSI wont change, but the speed that it travels will greatly increase in a smaller diameter barrel, it will just take a while to get your projectile up to supersonic speeds, so i would suggest a very long barrel, and thus a giant air tank, so as not to lose much PSI by the time it reaches the end of the barrel.

good luck!
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Unread postAuthor: DYI » Mon Sep 10, 2007 4:10 pm

The current plan is a 75 ci chamber running on 500 psi nitrogen, with either a 3/4" or 1" burst disk valve, and a 144" long .785 calibre barrel, firing a 10 gram projectile.

@ Scatdawg: one thing that you could help me with is piston valves. While the current cannon will use a burst disk for optimum performance, some of my larger cannons would be far more entertaining if they didn't take 15 minutes to reload. At some point, if it's okay with you, I could send you specs for a piston valve. Send me a PM if you want to discuss this more.
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Unread postAuthor: noname » Mon Sep 10, 2007 10:26 pm

I'm glad you aren't going to try a paintball; as they evaporate (well, just tur into a bunch of tiny pieces) at high velocities unless you freeze 'em. Good luck with the project, I'm looking forward to seeing the finshed gun.
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Unread postAuthor: joannaardway » Tue Sep 11, 2007 12:53 pm

noname wrote:I'm glad you aren't going to try a paintball; as they evaporate (well, just tur into a bunch of tiny pieces) at high velocities unless you freeze 'em.

I don't find that myself. My experience is that if they're wadded, they'll hold together pretty well at any speed I can get them to.
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Unread postAuthor: DYI » Tue Sep 11, 2007 3:06 pm

When I looked at the GGDT model and saw that the initial acceleration force was over 12 000 Gs, I decided against the paintball option.

The gun will be finished very quickly after the nitrogen tank comes, you can likely expect to see the finished product in about 1 month.
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Unread postAuthor: dongfang » Fri Sep 14, 2007 3:46 pm

Hi,

As for an inter-chamber piston, I would look for some kind of lightweight, disposable piston. A piston that only lasts one shot. Like the bottom third of a pop bottle (a cylindrical one, not Coca-Cola etc.). It will slam into the end of the primary cylinder (which is rubber padded?) and will have to be removed with a vacuum cleaner or something afterwards. But in high performance stuff, having some short life components is part of it..

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Unread postAuthor: DYI » Fri Sep 14, 2007 7:19 pm

The unfortunate bit about that idea is that when you do the work required to seal the piston with the walls of the tube, it isn't exactly easy to make anymore. There are a myriad of other problems with this design that I don't feel like delving into at the moment, but I really don't think it would work that well. I will do a two stage launcher at some point, but I can't see it happening in the near future, on my current budget.
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Unread postAuthor: dongfang » Sun Sep 16, 2007 4:09 am

Hi,

That´s of course right with the work....

Hey, there is sometimes a non-work solution waiting, but you have to find it first.

Example: My father once was doing a lot of caulking work on a boat, with some nonstandard tubes. He got tired of doing it manually, and wanted to hook it up to his pneumatic caulk gun in some way. Needed a piston. In the shower, he observed that the shampoo bottle looked like the diameter of the nonstandard caulk, and true enough! The bottom of the shampoo bottle did the job.

Thinking about that story, it was probably the first pneumatic cannon I ever saw: We amused ourselves with shooting the piston at a tarp.

So if you can find a pipe of _some_ kind that fits around a pop can or bottle, that is one step. Or detergent bottle. Or auto sprinkler fluid.. Or .. anything.

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